Gave 3 weeks notice, left in good standing, now old boss wants me to finish project?

[tranquility]

He was asked to do a favor...one that he had no legal obligation to do. He volunteered his time for the local PD. Happens all the time

One never has a legal obligation to do work. (Remember the slave thing.) I disagree with you this was a volunteer situation. This was work. Work under the threat of the PD going to the "legal department". The OP has the right to be paid for the work. The same work he did when he was a regular employee. While the normal wage rules don't apply to the government, I bet they would lose if the OP made a claim.